Tie the tenderloin with kitchen twine in 1 inch increments to form an even piece, so it will hold its shape during cooking.

Heat a large cast-iron or heavy skillet over medium-high heat. Season pork with 1 teaspoon salt and 1/4 teaspoon pepper. Sear tenderloin until well browned on all surfaces, about 1 1/2 minutes on each side including ends.

Transfer tenderloin to a cutting board, allow to drain and cool. Chill until ready to assemble and the pork is cold at least one hour or overnight.

In another large skillet, heat butter over medium heat. Add onion, cook until it softens, about 4 minutes. Add mushrooms, season with the remaining teaspoon of salt and 1/4 teaspoon pepper. Cook, stirring occasionally, until they are tender and liquid is released and evaporates, 8 - 10 minutes. Add sherry, cook until the mixture is dry, about 4 minutes more. Cool at room temperature or in the refrigerator for up to a day.

On a floured surface, roll the puff pastry into a rectangle 1/4 inch thick and big enough to enclose the pork. If using store-bought pastry, it may be necessary to lay out two pieces, overlapping, and roll them out into one piece. Spread the top of the tenderloin evenly with half of the pate, and spread half of the mushrooms evenly over the top.

Carefully invert the coated tenderloin into the middle of the puff pastry, mushroom side down. Spread another layer of pate on the sides and top of the tenderloin. Spread the remaining mushrooms over the top. Fold up the long sides of the dough to enclose the tenderloin, brushing edges with an egg-wash to seal. Trim ends if necessary, then fold up and seal. Carfully transfer the tenderloin seam side down, to a baking sheet, and chill at least 2 hours or overnight.

Heat oven to 425 degrees. Place a baking sheet on the middle rack of the oven until hot, about 15 minutes. Brush the top of the pastry with the egg-wash. Make 2 - 3 slits in the pastry to vent the steam. Sprinkle with sea salt if desired. Carefully transfer the pork Wellington to the preheated baking sheet. Bake until the pastry is golden brown, and the pork is done, approximately 60 minutes. Cover pastry with foil if it gets too brown during cooking. Let reston a cutting board for 10 minutes before slicing and serving.

(*) Most grocery stores have several kinds of pate to choose from; mushroom, vegetable, chicken liver, goose liver. It is your choice as to which you prefer.